Experimental And Theoretical Research On Constant Amplitude Fatigue Properties Of Weld Toe In Steel Tube Of Welded Hollow Spherical Joints In Grid Structures

Grid structures have been widely used in industrial buildings with suspending cranes.With the increasing of suspending cranes in quantity and size,the fatigue problem of grid structures has been more and more highlighted.Since the related theoretical and experimental researches are limited,fatigue design methods for grid structures are currently lacking at home and abroad.Hence,the applications of grid structures in industrial buildings have been seriously restricted.The fatigue of joints is very important to the fatigue of grid structures.Funded by Chinese National Science Foundation Project(51578357),the experimental and theoretical research on the fatigue properties of the weld toe in steel tube of welded hollow spherical joints in grid structures was conducted.The main research work and conclusions are as follows:1.The stress distribution of the welded hollow spherical joints was obtained by the static loading tests and finite element analysis.Then,the formula of the hot spot stress concentration factor of the weld toe in steel tube was derived.It provided the foundation for fatigue design methods of the weld toe in steel tube of welded hollow spherical joints on the basis of hot spot stress amplitude.2.The constant amplitude fatigue tests of 38 full scale welded hollow spherical joints were conducted using the MTS fatigue testing machine system.24 effective test data were obtained and fitted to a fatigue curve for the weld toe in steel tube by regession analysis.Based on the curve,the fatigue design methods were established by adopting the nominal stress and the hot spot stress respectively.3.The infrared thermal method was applied to the fatigue test of full scale welded hollow spherical joints.The fatigue damage position of the joint was predicted by analyzing the temperature rise of the surface of the joint.The cycle numbers were obtained when the crack penetrated the wall thickness of the joint.4.Macroscopic analysis of 33 fatigue fractures and microscopic analysis of 11 typical fatigue fractures by scanning electron microscopy were conducted.Morphological features of fatigue fracture of the weld toe in steel tube of welded hollow spherical joints were obtained.Then,the fatigue failure mechanism of the weld toe in steel tube of welded hollow spherical joint was revealed.5.By selecting the appropriate fracture mechanics parameters,Paris formula was used to estimate the fatigue life of the weld toe in steel tube of the welded hollow spherical joints.For the specimens with welding defects,the fatigue life could be estimated accurately.For specimens without obvious welding defect,the regularity is consistent between estimated fatigue life and experimental fatigue life.The research results of this paper will provide references for the revision of fatigue design methods of welded hollow spherical joints and promote the development of grid structures with welded hollow spherical joints in the industrial buildings.